Moon in lowest path of century may raise high tides worldwide

By
Robert C. Cowen /
December 30, 1986

THE United States National Ocean Service warns of record high tides around the world. But there's more than a potential for coastal flooding in the unusual motion of the Moon the NOS cites. It's an astronomical spectacle that inspired prehistoric ``scientists,'' and which anyone can enjoy. Our natural satellite is undergoing what the late Scottish archeo-astronomer Alexander Thom has called a ``major standstill.'' The Moon now is moving across the sky along the highest high and lowest low monthly paths it ever takes. It will continue to do so for much of 1987. This condition won't recur until the next century.

``Standstill'' refers to the orientation of the Moon's orbit around Earth with respect to Earth's orbit around the Sun. This orientation determines the difference between the highest and lowest paths the Moon takes through the sky each month and the highest and lowest paths the Sun follows in the course of a year. The Moon itself, of course, doesn't stand still. It continues to whiz along its orbit.

If the Moon's orbit around Earth were in the same plane as Earth's orbit around the Sun (the so-called ecliptic plane), Moon and Sun would follow the same paths across the sky. In winter, the Sun moves along a low path through the sky, especially as seen in middle latitudes. Six months later, in summer, it takes a high path. Its rising and setting points on the horizon are well north of where they are in winter. The Moon's monthly high and low paths and its most northern and most southern monthly rising and setting points would be the same as the annual extremes of the Sun were the orbits of Moon and Earth in the same plane.

But they aren't coplanar. The lunar orbit tilts about 5.15 degrees out of the ecliptic plane. This tilt makes the Moon's monthly extreme positions higher and lower than the summer and winter extreme positions of the Sun. To complicate matters, gravitational interaction of Earth, Sun, and Moon make the Moon's orbit rotate with respect to Earth's orbit. The practical effect of this is that the Moon's departure from the solar path varies through a regular 18.61 year cycle.

To put it another way, the difference between the Moon's most northern and most southern monthly rising and setting points on the horizon is generally greater than the comparable annual difference for the Sun. But that lunar difference goes through a 18.61 year cycle and we're at a point in the cycle where the difference is greatest.

The cycle seems to pause at this point. The lunar difference stays roughly the same for about a year before decreasing noticably - hence the term ``major standstilll.'' About nine years from now, the difference will be at a minimum - a condition Thom called a ``minor standstill.''

The present difference is dramatic. Watch the Moon throughout a month. For awhile around the full phase, it will arc on a high path through the sky. Half a month later, the old and new Moons will rise and set much farther to the South and will pass much lower through the sky.

Such a spectacle would surely have caught the eye of prehistoric people. Thom has shown how at least some ancient stone monuments in Britain could have been used to track the Moon and study the 18.61 year cycle. These ancient ``astronomers'' would have been interested in the Moon's motions for predicting eclipses. And, as is the case today, they would probably have been interested in the connection between the Moon and the tides.

The orientation of the lunar orbit with the greatest effect on tides occurs during a major standstill, as the NOS notes in its advisory. This is one of several factors behind the NOS tidal warning.

Other tide enhancing factors include the fact that, today (Dec. 30), the Moon reaches its most southerly point below the celestial equator (the projection of Earth's equator on the sky). The Sun also is well south of the celestial equator during the winter. Today, also, the Moon is at perigee - the closest its orbit comes to Earth.

Tomorrow, there's a new Moon, which means Sun and Moon line up to combine their tide-raising forces. Since the solar force is half the lunar tidal pull, this is a substantial combination.

Finally, Earth will go through perihelion, its closest approach to the Sun, Jan. 3.

NOS warns that these factors are combining in various ways to raise unusually high tides through Jan. 4.

In fact, we are in a period with the strongest astronomical tide raising forces of the remander of the century. A forecast made by Bernard Zeller at the Scripps Institution of Oceanography in 1983 predicted that the highest astronomically induced tides would occur between 1986 and 1990, with the peak occurring in 1987.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.